Method and circuit arrangement for improving carrier separation

Details Method and circuit arrangement for improving carrier separation Method and circuit arrangement for improving carrier separation

1997-11-17

2000-07-11

Kizou, Hassan

Multiplex communications

Generalized orthogonal or special mathematical techniques

Fourier transform

370203, 370208, 375354, 375355, H04L 2726

Patent

active

060883278

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method and a circuit arrangement for improving reception when digital signals are being transmitted.


PRIOR ART

In the case of the OFDM method (Orthogonal Frequency Division Multiplexing), one carrier is not used for each programme, but a large number of modulated carriers which are transmitted using frequency division multiplexing. The type of modulation may be, for example, QPSK, 64 QAM or else another digital type of modulation. The frequencies of the N carriers are at equidistant intervals f.sub.tr and are selected in such a manner that they form an orthogonal function system. The large number of carriers in this case results in a spectrum having a virtually rectangular shape, so that the OFDM signal has a constant power density in the band which is being used, similar to band-limited white noise.
The programme signals are decomposed into symbols of duration T.sub.s =1/f.sub.tr and are distributed over the N different carrier frequencies. In this case, a guard interval of length T.sub.g is assigned to each OFDM symbol, in which guard interval a part of the symbol is repeated in order to make the signal insensitive to echoes.
In order to separate the carriers from one another again in the receiver, a Fast Fourier Transformation (FFT) of length N is carried out, it then being possible to separate each carrier cleanly from the others provided the carriers are exactly orthogonal with respect to one another.
However, the carrier orthogonality can be disturbed by various causes. A frequency error in the carrier or spreading of the carriers can thus occur as a result of phase noise. The orthogonality is also disturbed if frequency elements are present which are located between the carriers. This can be caused by white noise or harmonic interference whose frequency does not coincide with a carrier frequency.
Furthermore, the wanted signal must be separated from the undesired adjacent channel signals by analog or digital filtering in the receiver. If the wanted signal fills the entire channel as far as the band limits, then the small separation between two adjacent signals requires very steep filters for separation, which require a high level of system complexity. Cutting off a number of carriers at the band limits in order to increase the separation can overcome this only to a limited extent.
Integration of all the frequency elements (leakage effect of the FFT) is carried out in both cases, which integration can lead to a considerable increase in the interference in the respective OFDM receiver.


INVENTION

The invention is based on the object of specifying a method for improving reception when digital signals are being transmitted, by means of clean carrier and channel separation.
The invention is based on the further object of specifying a circuit arrangement for use of the method according to the invention.
In order to improve the separation of the N carriers, the selectivity of the FFT filtering can be increased by enlarging the number of FFT components. However, this normally leads to an undesirably severe increase in the computation complexity, since the length of the FFT and thus the number of calculated coefficients must be an integer multiple of N. The refinement according to the invention of the time window which is used for the FFT makes it possible, however, to dispense with calculation of some of the coefficients. Depending on the transmitted signal and the transmission conditions, the implementation of the window can in this case be adaptively matched to the transmission conditions, with the aid of an estimation of the channel characteristics.
The channel separation can be improved if the signal is oversampled in the receiver, before the FFT. The selection of the oversampling according to the invention now makes it possible to dispense with calculation of some of the coefficients in this case as well and to use relatively simple analog filtering since steep filtering is then no longer necessary.
In the case of the carrier and channel separation, the FFT can be carrie

REFERENCES:
patent: 4794556 (1988-12-01), Rader
patent: 5357502 (1994-10-01), Castelain et al.
patent: 5396489 (1995-03-01), Harrison
patent: 5416767 (1995-05-01), Koppelaar et al.
patent: 5537435 (1996-07-01), Carney et al.

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